The invention relates to a single-piece and two-piece piston of an internal combustion engine and to a method for producing such a piston.
In order to comply with emissions limits, or to achieve emissions targets and fuel consumption targets, combustion temperatures and combustion pressures are being raised to optimize combustion, as a result of which the upper part of the piston in particular is subjected to severe thermal loads. The operating temperature of the piston in these internal combustion engines can exceed the acceptable limits of the piston material, in conjunction with the risk of thermal aging in which the alloy of the piston material loses strength and dimensional stability. To minimize the thermal loads on the piston, pistons with an integral annular cooling channel are used, in which a small amount of the lubricating oil of the internal combustion engine is sprayed through an injector nozzle as coolant, which flows through the cooling channel and then exits. DE 197 500 021 A1 discloses a cooling channel piston that includes an annular cooling channel in the region of the ring area radially offset to a surface area. The coolant flowing through the cooling channel acts to dissipate heat, where the effectiveness of this liquid cooling is determined by the volumetric flow of the cooling medium through the cooling channel. With the increasing specific power output of the internal combustion engine, known concepts for liquid-cooled pistons have to be optimized. It is, therefore, necessary to apply coolant selectively to further areas of the piston, in addition to an annular cooling channel. In order to implement this measure, DE 41 18 400 A1 shows an assembled piston that includes cooling slots with walls running parallel to each other, the slots starting from the cooling channel and running in the direction of the piston head.
It would be desirable to optimize the cooling effect of the piston upper part of a single-piece and two-piece piston in thermally highly stressed zones using a cost-effective measure.
Using the prior art as a starting point, the present invention provides a piston upper part of a single-piece and two-piece piston with integrated recesses.
The two-piece piston is configured as a liquid-cooled piston, consisting of a piston lower part and a piston upper part having a combustion chamber recess. These piston components are supported by joining lands, spaced apart radially from one another and forming a dividing plane and joined together using a material-to-material fit, such as by means of a welded joint, or frictionally, by means of a screw connection. Thus, the joined piston is assembled from a piston upper part and a piston lower part, for example, by means of a screw connection, or welded together, for example, by means of a welded joint. An annular cooling channel is introduced into the piston upper part, extending into the piston lower part and connected to an inner cooling space by connecting channels. To enlarge the cooling space, the piston upper part includes recesses connected to the cooling channel, oriented in the direction of the piston head and configured as a blind hole.
It is also possible that the liquid-cooled piston of an internal combustion engine consists of a piston lower part and a piston upper part having a combustion chamber recess, where the piston is configured as a single-piece piston without a dividing plane.
To address the problem, the at least one recess emanating from the cooling channel and introduced circumferentially in the piston upper part is shaped such that its walls widen conically as they rise. Because of the resulting spreading out of the walls, a maximum cross-section is achieved in the areas of the greatest depth of the recess. While retaining specified wall thicknesses compared with previously known solutions, the invention enlarges the cooling space through which the coolant flows, being expanded at the bottom, and thus optimizes cooling of the piston upper part. One shape for the recess under the invention, spaced apart from a central contour, follows the tub-shaped combustion chamber recess in the piston head. In conjunction with a large-capacity space in the recess that increases the coolant intake, the “cocktail shaker” effect can be enhanced, and consequently the cooling effect can be augmented. The size and the extent of the recess that forms an expansion of the cooling channel is not limited by design specifications, for example, the location and arrangement of the dividing plane between the piston lower part and the piston upper part or the cooling channel, but can expand, for example, specifically in the direction of the combustion chamber recess. The recesses are intended for piston upper parts with a relatively small combustion chamber recess in order to cool the resulting large wall thicknesses and material accumulations in the piston head optimally. Coking to the point of burn-off and reduction in material strength can be avoided in this way. In conjunction with the measures to optimize cooling of the piston upper part, emissions requirements (Tier 3 and IMO) for assembled pistons with a small combustion chamber recess diameter can be met. The possibility exists of combining the piston upper part formed in accordance with the invention with existing proven piston lower parts. With a liquid-cooled piston configured as a single-piece piston of an internal combustion engine, the type of piston lower part and piston upper part is equally suitably shaped, as described previously.
In accordance with the invention, the size and extent of the recess is not restricted by the external diameter of the joining lands or of the supporting surfaces in the area of the dividing plane between the piston upper part and the piston lower part. Rather, the measure under the invention makes it possible to extend the recess intended for cooling as far as the thermally highly stressed zone. The cross-sectional profile in the recess bottom consequently exceeds the cross-sectional profile in the area where the recess passes into the cooling channel because of its conical widening. The piston head includes several recesses positioned distributed around the periphery and connected to the cooling channel. These recesses, configured as a blind hole, which selectively enlarge the cooling space, bring about improved, efficient cooling of the piston upper part. The recesses result at least locally in reduced wall thicknesses in the piston upper part, compared with the combustion chamber recess, the ring area, the upper land and the piston head. Contingent upon matching wall thicknesses between the recesses configured in accordance with the invention and the adjacent thermally severely stressed zones, a structurally strong piston upper part is realized that can meet the most severe demands.
The measure under the invention reduces component temperature to a level below the flame point of conventional cooling oils, simultaneously reducing the risk of coking for the lubricating oil of the internal combustion engine that is used as the coolant. In addition, there is no risk of an insulating oil carbon layer forming that reduces the cooling effect or of detrimental thermal piston deformation because of reduced strength in the piston material. Through the decisively improved heat dissipation and thus cooling effect of the recess under the invention, the piston upper part and consequently the entire piston can be used for higher combustion temperatures and compression pressures, i.e. in internal combustion engines with a high power density. In addition, the large-capacity shape of the recesses, which can be produced cost-effectively, reduce the weight of the piston upper part, particularly with small combustion chamber recess diameters.
One aspect of the recesses, which widen conically over their longitudinal extent, provides for the recesses to be shaped specifically as slots, drilled holes or channels distributed around the periphery in the piston upper part. Lands formed from the material of the piston upper part are provided between the recesses and the cooling channel. As an alternative to the lands, walls or supporting ribs can be used, where the walls or supporting ribs differ from the lands in their respective shape. To achieve high structural strength for the piston upper part, one solution is to configure the conically expanded recesses as torte-shaped cooling space chambers with a honeycomb structure that simultaneously has a positive effect on cooling properties and results in a greater cooling surface. In addition, the cooling space is expanded as a result.
In accordance with a further design aspect, the adjacent recesses are introduced opposite each other in the piston upper part alternately in geometric sizes that are identical to or differ from each other and/or inclinations to each other. This measure enables a selective extension of the recesses right into thermally highly stressed zones without the risk of weakening a component.
In accordance with a further aspect, the walls of the recess are oriented inclined at an angle of “α, β” between 0° to 40°, preferably of ≦15° to a piston longitudinal axis, to achieve consistent wall thicknesses as far as possible with respect to the thermally highly stressed zones. Matched to the design construction of the piston head, a further possibility is to design the angle of inclination of oppositely located walls, in particular of an inner wall and an outer wall, to be consistent with one another or to diverge from one another.
For cost-optimized production and to prevent component weakening, provision is made to give the respective conically spreading recesses a rounded bottom that has a positive effect on structural strength. A radius “R” between 1.5 mm and D/2 (D=maximum diameter of the tool, for example a milling tool, used for the metal-cutting process for the rounded contour) is preferably used. As an alternative, it is possible to give the recess bottom a double-rounded contour, forming a dome-shaped, arched depression. In addition, to match the design construction of the piston head, the double-rounded recess bottom can have a stepped shape. The recess bottom can have a pronounced undulating surface, which produces an enlarged surface, or a finely undulating surface. Through additional machining steps in production, for example, milling cuts, the stepped transition between the curvatures is reduced, the result of which the surface of the curvature is qualitatively improved by reduced undulation on the surface and the size of the surface is reduced. It is thus possible by making a greater number of cuts to produce a finely undulating surface. As an alternative to a rounded final contour, a chamfered configuration for the recess bottom is provided. The recesses in the cooling channel additionally create increased swirl in the cooling channel. By adjusting the surface of the recess bottom and reducing the diameter of the recess bottom it is possible to reduce, or optimize, emissions during piston operation. Emissions during operation can also be reduced by modifying the stepped transition. By varying the depth of the recess, viewed from the piston lower part in the direction of the piston upper part, the cooling space can be adjusted to the shape of the depression in the combustion chamber recess. In the case of a single-piece piston, it is possible to adjust and design the recess bottom by means of the shape of the casting mold member, the shape of which is the negative of the shape of the recess bottom in specific areas.
A further of the invention provides for arranging the recesses, which are configured as channels, drilled holes or slots, symmetrically or asymmetrically around the periphery in the piston upper part. The position, orientation and size of the recesses can be adjusted to the different thermal loads. For example, it is possible to design the cooling space volume, or cross-sectional volume of the recess, on the pressure side differently compared with the cross-sectional volume on the counter-pressure side of the piston upper part. The location and design of the recess is located and designed in such a way that any material weakening in the piston upper part is avoided.
In accordance with a method under the invention, the following steps are followed to produce the recesses. First, a casting mold member, such as a salt core, matching the shape of the recesses is anchored in position in the casting mold intended for the piston upper part. After casting and chilling the piston upper part, the casting mold member is removed by purging.
In accordance with a further alternative method, the following steps are followed to produce the recesses for a single-piece piston. First, a casting mold member, such as a salt core, matching the shape of the recesses is anchored in position in the casting mold intended for the single-piece piston that has a piston upper part and a piston lower part. After casting and chilling the piston upper part, the casting mold member is removed by purging.
A further alternative method for producing the recesses provides for mechanical, three-dimensional machining work. Turning and milling work are preferably suitable by which cavities forming the recesses are introduced in the piston upper part. In addition, it is possible to produce the recesses by means of milling or using drilling tools.